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NSA (National Security Agency) "Eavesdropping On Hell", by Robert J. Hanyok

Historical Guide to Western Communications Intelligence and the Holocaust, 1939-1945

Chapter 2: Overview of the Western Communications Intelligence System during World War II

Step 2: Intercepting the Messages

After the requirements had been set, the next step was intercepting the radio terminals that were designated the highest priority targets. It was at this juncture in the communications intelligence system that the general requirements had to be translated from the vague entries such as “German military” to a list of targeted radio stations that could be monitored by Allied radio intercept personnel. Before discussing the allotment of target stations to monitoring stations, it is necessary to describe the communications the Allies monitored. There were two major types of communications media that were targeted: cable (or wire) and radio. Both had their advantages and shortcomings.

The first of these, cable intercept, proved to be a useful but limited source of communications intelligence. At the war’s outset, both the United States and Great Britain invoked wartime information contingencies and imposed restrictions on telegram and cable traffic to, through, and from both countries. All such wire traffic - personal, commercial, and diplomatic - was subjected to censorship review. Each resident foreign diplomatic mission was required to submit a copy of each message it sent, even in its encrypted form, to the appropriate government censorship office. The main reason for this was to control the flow of information out of the country that was critical to the war effort. [29] In England, the General Post Office over- saw this program, sometimes referred to as “traffic blanketing.” [30] The British also implemented censorship practices at all overseas Commonwealth cable terminals located in such places as Malta, New Delhi, Gibraltar, and Barbados. [31] In the United States, the government, through the Office of Censorship run by Byron Price and staffed by civilians and contingents from the Army and Navy, received copies of all such wire traffic (before transmission overseas or upon receipt from an overseas terminal) from the various cable and wireless telegram companies such as Mackay Radio, Western Union, Radio Corporation of America, and Global Wireless. [32] These companies had terminals in New York, Washington, DC, New Orleans, Louisiana, Galveston, Texas, and San Francisco, California. Copies of cable traffic that passed through overseas U.S. and British terminals on to other countries were also collected under the wartime restrictions. This coverage affected mostly diplomatic, personal, and commercial cable traffic between Europe, the United States, Latin America, and Africa.

Right: Copy of Netherlands cable from U.S. Censorship Office

This cable intercept had the primary advantage of simplifying the intercept problem. All messages flowing through an American or Commonwealth terminal were accessible to Allied cryptologists. Another advantage was the errorless copy of the intercepted cable message; each message usually was free of transmission garbles and missed cipher groups that could hinder exploitation, except for errors by the originator’s code clerk. The collection of cable messages was inexpensive for the cryptologic agencies since the national censorship offices performed the task.

On the other hand, there were limitations to this collection method. For example, only cables sent between Washington (and other U.S. terminals with overseas connections such as San Francisco, New Orleans, and New York) and an overseas terminal were available to the Americans.

Copy of Netherlands cable from U.S. Censorship Office

Messages sent on cables not routed through American- or Commonwealth-controlled terminal stations were beyond intercept. There were few means then to obtain these cables except through covert means, such as placing taps or suborning cable clerks. These methods risked eventual exposure and were difficult to hide and continue without discovery by foreign security agencies.

Another difficulty was that some of the targeted countries knew or suspected that the Allies probably were reading their cable traffic. To defeat the intercept and possible exploitation of their cables, some diplomats were warned by their capitals to take certain precautions when sending cables, even encrypted ones. For example, Axis diplomats in Europe and the United States were told to be careful when they included excerpts of public speeches and newspaper stories in their messages. They were warned not to quote them verbatim because the excerpts were also available to Allied code breakers and could be used as “cribs” to break the enciphered messages. [33] To defeat the Allied cable censors, some countries sent sensitive correspondence by diplomatic pouch or courier. Of course, the courier pouches could be searched covertly (“tossed” was the expression) by Allied agents. But this technique was difficult to arrange and, if discovered, was a possible source of diplomatic embarrassment. It appears that Allied agents probably did this anyway. In early 1942, the U.S. Office of Strategic Services, and its predecessor, the Office of the Coordinator of Information, received copies of diplomatic messages from the British Security Coordination. A number of these messages appear to have been obtained in this manner. [34] (See pages 65-6).

In occupied Europe, Nazi military and occupation authorities made use of cable telephone or telegram systems where available, thus denying the Allies access to these communications. Also, during the years of the war, the Germans, especially the military, expanded, repaired, and improved the European cable backbone network in occupied Europe in order to support their operations. In four years, the German army nearly doubled the size of the European cable trunk network to 6,900 miles. [35] This cable expansion denied more intercept to the Allies.

The other and much larger and lucrative source of Allied intercept was radio communications. Since its introduction at the turn of the century, radio communications had expanded dramatically. As technology advanced and industry was able to mass-produce components and radio sets, the speed and distances that information could be transmitted grew. Many advanced countries had developed complete national-level communications networks that served private and public functions. When the war began, governments quickly took over many facilities and converted them to military or other official uses. The use of these in place systems presented more problems for Allied intercept stations since they had to match Axis capabilities.

Many Axis and neutral diplomatic missions, not needing or trusting the Allied-controlled cable systems, transmitted their diplomatic messages by long-range, high frequency (HF) radio. [36] Axis naval and air force units and organizations relied on long-range HF radio for their communications, unless ground-based cable systems were available. Some communications operated on lower frequency bands similar to those used by commercial broadcast stations. Many Axis units used low-power high frequency radios to communicate with their local commands that reduced the chance for interception by Allied monitoring stations.

The Axis used different modes of communications on the many frequency bands. The most common mode was manual Morse, the famous “dit-dah” or on-off keying of a continuous wave signal. This mode appeared on all frequencies, and all service and government elements used it. The Axis also used teleprinter systems for high- volume traffic among command centers, though this appeared later in the war. Voice communications was also heavily used. Most often this appeared at higher frequency ranges. The most common users were aircraft pilots communicating among themselves and with ground controllers. Some aircraft and ground stations used long-range HF voice systems, though this was rare because of the lack of security. [37]

German Police units operating in the western USSR used both long-range high frequency and the shorter-distance low frequency and medium frequency manual Morse communications to send reports about their activities from major command centers in Minsk and Kiev to headquarters back in Berlin. Police battalions received radio or courier reports from companies and detachments in the field. The battalions, in turn, sent their radio reports over HF radio to regimental headquarters in the region in which they operated. [38]

For the Allies, intercepting Axis and neutral radio communications of interest presented a myriad of problems. The most obvious problem, as mentioned earlier, was simply the number and location of Axis radio terminals - perhaps several thousand. In order just to hear these radio stations meant that the Allies had to develop extensive corps of thousands of radio intercept operators to man the monitoring stations to collect the Axis and neutral communications for intelligence.

The major Allied radio intercept organizations were, for the British, the Radio Security Service (belonging to M.I.6), the General Post Office, and those units of the various services of the British armed forces, known as the “Y- Service.” For the Americans, the radio intercept personnel were assigned to the Signal Intelligence Service (Army) and organized under the Second Service Battalion, OP-20-G (Navy), the United States Coast Guard, the Office of Strategic Services, and the Federal Communication Commission. Canada operated the Examination Unit of the Canadian National Research Council. A number of men and women from other Commonwealth countries also performed as intercept operators. These included personnel from Australia, New Zealand, and India. Commonwealth military personnel who performed intercept services were organized into units known as Special Wireless Groups. Additionally, contingents from smaller Allied forces also intercepted Axis communications.

All of these personnel were stationed at monitoring facilities located around the world. These facilities ranged from large field stations, such as at Beaumanor, Leicestershire, England, or Vint Hill Farms in Virginia, to small direction finding huts in Alaska or Scotland. It is difficult to get a precise total number of Allied monitoring sites. Many stations did exist for the duration of the war. Some served only to support a campaign. Many stations were limited to a direction finding function only. Some sites, such as the U.S. Navy station at Muirkirk, Maryland, originally were commercial radio stations that were taken over by the military. American sites, including those of the Army, Navy, Coast Guard, OSS, and FCC numbered over seventy-five. British and Commonwealth stations numbered around sixty and were located in the British Isles and overseas in Australia, Ceylon (Sri Lanka), Egypt, Palestine, Malta, Iraq, Kenya, and India.

Generally, Allied overseas monitoring stations supported the commands in the theater of operation in which they were located. For example, in early 1942 U.S. Army radio intercept companies that were assigned to locations in the British Commonwealth, such as in Northern Ireland, or in the Atlantic region, at sites in Iceland and Newfoundland, intercepted German military, naval, or espionage communications. [39]

An intercept section at MS-1, Vint Hill Farms, Virginia

Above: An intercept section at MS-1, Vint Hill Farms, Virginia

In the Pacific and Asian theaters, some field stations were manned by a mix of personnel from the United States and Commonwealth countries, such as the large sites at Central Bureau Brisbane and the Wireless Experimental Center in India. Other facilities were manned almost exclusively by personnel from one nation, such as the Americans at the U.S. Navy radio intercept site in Hawaii and the British in Ceylon. On the other hand, in the Southwest Pacific area of operations, Australian detachments accompanied SIS units to forward intercept sites in New Guinea and the Philippines to support General MacArthur’s advance. [40]

Intercept operations of USA Signal Radio Intelligence Company, Fifth U.S. Army in North Africa (Courtesy: National Security Agency)

Left: Intercept operations of USA Signal Radio Intelligence Company, Fifth U.S. Army in North Africa (Courtesy: National Security Agency)

Most field sites were controlled and manned by one service element: army, navy, air force, or security service, such as M.I.6 or the OSS. This arrangement was not absolute; other service contingents or nationalities could be present. Also, intercept missions could cross service boundaries and responsibilities. For example, early in the war, U.S. Navy sites did collect diplomatic communications. But service priorities usually prevailed when it came to mission assignment. For example, a tentative 1943 intercept assignment list for the major SIS monitoring station #1, located at Vint Hill Farms, Virginia, was dominated by international diplomatic and commercial terminals.

The number one assignment block included all government traffic between Tokyo and Rome, Bern, Vichy, Ankara, and Stockholm. The second and third tier priority was both German and Italian weather traffic (known as “WX”). The fourth priority was all government traffic to and from Berlin to Manchukuo, Madrid, occupied China, and Lisbon. Interestingly, the only military traffic assigned to Vint Hill Farms was that of Germany and it was reserved for the lowest priority. [41]

Eventually, the assignment of targets by city (or “circuit”) and specific types of messages was dropped because it was considered inefficient. Too often, there was needless duplicative copy of the same radio traffic. In mid-1943, the SIS adopted the procedure of tasking by frequency and terminal callsign. This targeting method was similar to that used by the British when tasking their field sites. [42] The British also pioneered the central control of all collection sites. From Bletchley Park, GC&CS could direct the activities of all the British intercept sites, though its measure of control over service monitoring stations may have been less certain. [43]

By early 1944, the American and British sites were experimenting with a joint and centralized authority that controlled collection of important target networks. Field sites informed a control authority by teleprinter of current targets being collected. The control station then assigned any remaining priority target terminals that were not being monitored. Field sites also sent in weekly reports on their coverage that allowed refinement by oversight committees in both countries, the Y-Committee (later Y-Board) in Great Britain, and at Arlington Hall. [44]

Still, even with the number of intercept and direction finding facilities that eventually would be built during the course of the war - over 130 - the Allies would never have enough monitoring positions and radio receivers to match the number of Axis transmitters and the volume of radio traffic they sent. Precise numbers on messages transmitted by the Axis and those intercepted by the Allies are not readily available. One estimate is that a Japanese area army command could send as many as 1,400 messages a day. [45] Some educated estimates by veterans and scholars of Allied wartime cryptology tend to support the contention that Allied intercept never resembled the popular analogy to a vacuum cleaner in which all Axis traffic was “scooped up in the ether.” If anything, Allied intercept more closely resembled a form of “sampling.” There is one estimate that the U.S. managed to intercept only 60 percent of all Japanese naval communications in the time leading up to and at the time of the Japanese strike at Pearl Harbor. [46] In rare cases, where the high-level interest existed that could focus resources, more complete coverage could be arranged. During the United States-Japan negotiations leading up to Pearl Harbor, all but a few of the messages between Tokyo and its embassy in Washington were intercepted. But such thorough coverage was not the norm.

Besides the large difference between the level of Axis communications and the Allied monitoring capability to collect it, there were a number of other technical limitations that hindered complete and effective Allied intercept operations. Atmospherics, such as local weather and the presence of sunspots, affected reception of communications, especially those on the high frequency range, which was the most commonly used transmitting frequency band. The location of monitoring stations was critical to hearing enemy transmissions. The sheer distance between the transmitter and the monitoring station could determine whether the latter could hear its target. The time of day and the season could also influence the range and clarity of communications. At night, the composition of the layers in the atmosphere changes, producing two effects: increasing the range at which radio communications can be heard, while creating nonreception areas, known as “skip zones.” The local topographic conditions in which a radio transmitter was located played a role in how well a radio signal could be heard at an intercept site. Mountains, heavily forested terrain, or jungle dampened radio transmissions, while deserts increased the range of a transmission. The frequency and power output of the transmitting terminal also mattered. The sensitivity of the monitoring equipment at an intercept site was a crucial element.

All of the above environmental factors were considered when the Allies selected locations for their monitoring stations around the world. An important part of this process was the work of teams that performed field studies of potential sites. The results of their surveys determined the location of the monitoring stations. [47] One of the most productive Allied intercept stations against Japanese military communications originating from Japan was the U.S. Army’s site at Two Rock Ranch outside of Pentaluma, California. An U.S. Army site that specialized in intercepting international diplomatic communications was located near Asmara, Ethiopia.

At the monitoring station, the capabilities of individual intercept operators could make a distinct difference in what was copied or missed - the difference between so-called “clean” copy usable by cryptanalysts or linguists, or that riddled with missing or mistakenly heard characters or groups that hindered recovery of message text. The ability of Allied intercept operators varied according to training and experience. The training of these specialized personnel included general skills in communications procedures and modes of transmission, as well as training tailored towards specific communications targets, such as instruction in the difficult Japanese telegraphic kana code. As intercept operators gained experience, their “copy” would become usable.

Two sites primarily intercepted German Police communications. The first was the British Army Y-station at Beaumanor in Leicestershire, located about eighty miles northwest of London, England. Beaumanor was one of the largest British intercept sites and operated with 140 radio receivers by the end of the war. The primary mission of Beaumanor was German military communications and included that of the German Police. Because of the vast distances between the operational police units and their various headquarters hierarchy that extended back to Berlin, a variety of radio frequencies were used. These extended from the low frequency band associated with commercial broadcasts up to the standard high frequency bands used by the military.

The site was able to copy police messages from Russia on all of these frequencies. In fact, once the initial recovery of the police radio network was completed, that is, identifying all of their stations, operating frequencies and schedules, the collection effort was not too difficult to maintain, despite changes by the Germans. In early September 1942, the police changed their callsigns, but not their frequencies or schedules. The Y-operators at Beaumanor were able to quickly equate the new and old callsigns within a few days. [48]

The other allied monitoring station that copied police and SS communications merits a short consideration because of its unique contribution to the intercept of Axis communications relating to the Holocaust, as well as the curious story of the site itself. P.C. Cadix (P.C. stood for Poste de commandement or command post) was a covert Allied intercept site located in the south- eastern part of unoccupied Vichy France at the Chateau de Fouzes near the town of Uzes. The site was manned by a polyglot team of Poles, exiled Spanish Loyalists, Free French, and the occasional stray Englishman. The monitoring station originally had been formed in late 1939 by Colonel Gustave Bertrand, head of the French Army’s radio intelligence organization, the section d’ examener (S.E). In its initial configuration the site was first known as P.C. Bruno located near Paris, France.

Following the fall of France in June 1940, the team that had manned P.C. Bruno, after spending a short exile in Algeria, returned secretly to southern France. From October 1940 to November 1942, P.C. Cadix, as the site now was named, intercepted German high frequency communications in occupied France and elsewhere. Because of the peculiarities of HF radio propagation mentioned above - such as the interval of the angle of reflection off the ionosphere of the sky wave of a radio signal, known as the skip distance, and extended propagation of HF and lower frequency bands during the night - P.C. Cadix was able to monitor German radio traffic from the Russian front at night, and signals from Germany to the units during daylight. Because of this situation, Beaumanor and P.C. Cadix split the work on German Police traffic, with the British copying on even days and the French site working the police nets on odd days. [49] Among the radio messages intercepted by Bertrand and his team were over 3,000 sent by German Police and the SS formations, many of which contained reports of the atrocities committed by these units. [50] P.C. Cadix transmitted the decrypts by radio to Bletchley Park. Ironically, these transmissions were encrypted with an Enigma. [51] Cadix continued to operate until late 1942 when it was forced to shut down, and its staff had to flee pursuing German security forces during the occupation of Vichy France that followed the Allied invasion of French North Africa.

It is a most important point to understand that the intercept of an Axis message was the initial step in the communications intelligence process. The quality of the intercept pretty much defined whether that message eventually could be useful for Allied intelligence. Radio reception conditions and the abilities (or lack thereof) on the part of monitors dictated the duration and difficulty of the subsequent analytic exploitation of intercepted messages, or even if the analytic effort could be started at all.


[29] War Department Memorandum, October 19, 1942, “Experiment in Intercepting Cable Transmission.” NARA, RG 457, Box 1276, “Secrecy of Radio and Cables.”

[30] Information on the censorship mission of the General Post Office is located in pieces HW 53/10 to HW 53/12.

[31] “Report of Colonel McCormack’s TDY to London,” June 1943, Tab B, “Berkeley Street,” 1.

[32] Initially, the U.S. Army and Navy performed this censorship mission. It was assumed by the Office of Censorship, which picked up the military censorship programs. This censorship also included all mail entering and exiting the United States. Telephone calls, whether over cable or radiotelephone, also were monitored, For a history of the Office of Censorship, see Michael Sweeney, Secrets of Victory: The Office of Censorship and the American Press and Radio in World War II (Chapel Hill, North Carolina: University of North Carolina Press, 2001). Of interest is that the eventual commander of the SIS, Colonel Preston Corderman, began his wartime service in the Office of Censorship directing and training postal censors.

[33] For example, see Lisbon (Foreign Minister) to Washington, 22 June 1944, MC-916, in which the Washington embassy is reminded to paraphrase public text, such as speeches, so as to avoid the use of them as plaintext cribs. Also, Washington (Hopenot/Baudet) to Algiers, 13 May 1944, MC-806, in which the French Committee for National Liberation is reminded that all cables can be obtained by the “enemy” and exploited because the cables were encoded with Vichy codes. RG 457, HCC, Box 879, “Code Instructions.” Also, Tokyo (Shigemetsu), 9 August 1943, warning that the Allies may purposely print news stories that diplomats would quote and thereby could endanger Japanese codes because plaintext would be known. NARA, RG 457, HCC, MND Translation #93904. Finally, Berlin (Oshima) to Tokyo, 5 March 1944, CI-1813, in which the Japanese diplomats in Germany discreetly instruct Tokyo to keep new communications routes secret from the British “telegraph office.” RG 457, HCC, Box 954, folder 2863, “Japanese Message Translations Categorized as CI (Code Instructions) for Diplomats, 1943-45.”

[34] “Intercepted Diplomatic Messages,” NARA, RG 226, Entry 210, Boxes 400, 402, 406 , et alia.

[35] Kenneth Macksey. The Searchers: Radio Intelligence in Two World Wars (London: Cornell University Press, 2003), 108-9.

[36] Radio frequencies are part of the electromagnetic spectrum. The most used frequency bands during World War II were very high frequency (30-300 Megahertz), high frequency (3-30 Mhz), medium frequency (300 kilohertz – 3 Mhz), and low frequency (30-300 Khz).

[37] During the war, in 1943, the Allies developed and fielded a secure speech unit for use on HF radiotelephone circuits among the Allied commands and capitals. The system, named SIGSALY, was contained in an equipment hut that weighed 55 tons. For more on this see J.V. Boone and R.R. Peterson, The Start of the Digital Revolution: SIGSALY. Secure Digital Communications in World War II (Fort George G. Meade, MD: Center for Cryptologic History, 2000).
[38] NARA, RG 457, HCC, Box 202, “Study of German Police Communications.”

[39] “Radio Intelligence Companies, Locations, 3-20- 42.” SRH-145, “Collection of Memoranda on Operations of SIS Intercept Activities and Dissemination, 1942- 1945.” 6.

[40] Geoffrey Ballard, On ULTRA Active Service. The Story of Australia’s Signals Intelligence During World War II. (Australia: Spectrum Publishing ltd., 1991), 238-252.

[41] “Directive for Station 1, listed in order of priority (tentative).” 30 January 1943. RG 457, HCC, Box 1432, “SSS Intercept Priorities Memorandum.”

[42] “History of Intercept Control,” June 1944, NARA, RG457, HCC, Box 1128, Folder 3634, “History of Intercept Control.”

[43] This control was not without a struggle. Several times early in the war, service intelligence heads tried to limit GC&CS control through bureaucratic maneuvers in the Y-Board, a committee under the JIC that managed intercept operations. See Hinsley, Volume II, 23-4 and Macksey, 129. Also PRO HW 14/30, “GC&CS Directorate, Policy Papers,” “No. 6. I.S.,” 1 March 1942.

[44] Memorandum from Williams, “International W/T Services. Cover in the U.K.” 21 January 1944, and “History of Intercept Control,” June 1944, NARA, RG457, HCC, Box 1128, Folder 3634, “History of Intercept Control;” also see Macksey, 160-2; SRH-349, 15-16.

[45] Edward Drea, MacArthur’s Ultra: Codebreaking and the War against Japan, 1942-1945. (Lawrence KS: University Press of Kansas, 1992), 52-53.

[46] Philip H. Jacobson, “A Cryptologic Veteran’s Analysis of Day of Deceit.” NCVA Cryptolog, (Vol. 21, No. 1, Winter 2000), 3.

[47] See RG457, E9032, Box 584, Folder “Optimum Intercept Locations, 1943” 1943 1201 and Box 1382, Folder “Choice Site for Station AY,” 1944 0328.

[48] “German Police and SS Traffic,” NARA, RG 457, HCC, Box 1386.

[49] PRO, HW 14/11, “Report on German Section No. 4, I.S.,” 2.

[50] Bertrand, 118.

[51] Ibid., 129-30.


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